1. Field of the Invention
The invention relates to the field of scanning microscopes, and more particularly to those microscopes that employ a fine probe which senses shear forces close to the surface of the object being scanned.
2. Art Background
Near-field spectroscopy is a very promising technique for imaging semiconductors. In the context of imaging, near-field means that spatial resolution is less than .lambda./2. Diffraction limited imaging, by contrast, provides spatial resolution that is greater than .lambda./2. In near-field imaging, light is transmitted through an imaging probe to the surface. The probe-sample separation distance is less than the requisite spatial resolution. The light reflected from the surface is scanned by the probe and transmitted back to a detector. The probe oscillates in a plane substantially parallel to the surface of the scanned sample. A photodetector and an optical imaging means are used to detect changes in the oscillation of the probe tip. The change in probe tip oscillation is used to analyze the topography of the sample surface.
Semiconductors are spectroscopically imaged using techniques such as luminescence, reflectance, absorption, transmission, photoconductivity, and Raman scattering. In these techniques, an image of a surface is obtained by analyzing an optical signal from the surface. The strength of the optical signal determines the quality of the optical image. Therefore, a weak optical signal will generate an image which is inferior to an image generated by a stronger optical signal. The light losses associated with a particular imaging technique, or an apparatus used to practice such a technique, affect the strength of the optical signal used to generate the image. The greater the light loss, the weaker the optical signal.
For example, in a luminescence imaging system, the source light is absorbed by the surface being imaged, the photoexcited surface emits luminescence and reflected light, and the emitted light is then imaged. Therefore, the emitted light must be a collectible signal for it to be used to generate an image. The near-field luminescence technique utilizes a probe with a sub-wavelength sized aperture. The intrinsic loss of photons is on the order of 1.times.10.sup.5 in a fiber probe with an aperture size on the order of 1/10 of a wavelength. Although the light losses associated with aperture size are reduced by increasing the aperture size, an increase in aperture size will decrease the resolution of the resulting image. The near-field luminescence technique also probes in a very small spot size, which also causes a loss of photons. For spatial resolution on the order of 1/10 wavelength, the penalty is on the order of 1.times.10.sup.2 compared to diffraction limited imaging.
The conversion of the source light to collectible luminescence also results in the intrinsic loss of the source light. These losses are on the order of 1.times.10.sup.5 to 1.times.10.sup.7 in luminescent imaging systems. Shadows east by the probe in the near-field luminescence technique also reduce the signal level. A near-field apparatus which reduces the photon losses associated with the near-field spectroscopic imaging techniques without a commensurate increase in the aperture size of the probe is therefore desired.